1. Field of the Invention
This invention relates in general to drywells and more particularly to a drywell structure which utilizes a filtration principle to prevent silt and other foreign materials from being transported by the drain water into the subterranean levels.
2. Description of the Prior Art
As is known in the art, a drywell is a structure formed in the earth for receiving water such as from a street after a storm, a roof drain system, floor drain system, or the like and draining that water into a permeable subterranean soil. Typically, a drywell is drilled, or otherwise formed in the ground so as to extend from ground level into permeable soil and the drywell is backfilled with selected rocks. A settling chamber defined by a concrete liner is located in the upper end of the drywell with a manhole cone on its upper end. A cast iron ring and grate are provided in the top of the manhole cone to receive the water to be drained, and the otherwise open bottom end of the setting chamber is closed with a porous filter fabric. In some instances, the lower portion of the concrete liner is provided with drain holes that extend laterally from the settling chamber into the rock-filled drywell. An imperforate overflow pipe extends upwardly from the bottom of the settling chamber and a cylindrical large mesh debris screen extends coaxially upwardly from the open upper end of the overflow pipe. A preforated drain pipe depends from the lower end of the overflow pipe through the rock-filled drywell and an injection screen is mounted on the lower end of the drain pipe.
When drain water is received in the drywell structure, it will enter and fill the settling chamber to the level of the open upper end of the overflow pipe. When filled to this level, drainage will begin in an overflow manner via the overflow pipe, drain pipe and injection screen through the rock interface into the permeable soil. When and if the permeable soil becomes saturated, water flow through the injection screen will be reduced and the flow rate will be increased through the multiple apertures of the perforated drain pipe. And, water will also flow out of the settling chamber through the filter fabric provided in its bottom end and through the lateral drain holes formed in the lower portion of the concrete liner.
As in most, if not all, water drainage, some silt and other foreign matter will be carried by the water into the settling chamber. The ideal result will be that the silt and non-floating foreign matter will settle to the bottom of the settling chamber and the floating foreign materials will be prevented from entering the overflow pipe by the debris screen, and periodic cleaning of the settling chamber is needed to remove the settled materials.
Even when the settling chamber of such a prior art drywell is clean, not all of the silt and other foreign materials will settle out, particularly when incoming water flow is heavy such as during and immediately after a storm. As the settling chamber becomes filled, this problem is aggravated and can become critical if the required cleaning of the settling chamber is neglected.
The silt and other foreign materials which enter the overflow pipe and are carried to subterranean levels result in contamination of the drywell which at first reduces its water dissipation capabilities and eventually destroys the useful life of the drywell. The silt and other foreign materials form a cake which clogs the porous soil around the drywell and clogs the rock back fill. When this occurs, the drywell must be abandoned.
Therefore, a need exists for a new and improved drywell structure which overcomes, or at least substantially reduces, the problems and shortcomings of the prior art.